


Always

by Anonymous



Category: Haikyuu!!
Genre: Anal Sex, Brother/Brother Incest, Explicit Sexual Content, Fantasizing, M/M, Masturbation, Oral Sex, Sibling Incest, Twincest
Language: English
Status: In-Progress
Published: 2020-09-19
Updated: 2020-09-27
Packaged: 2021-03-08 03:35:03
Rating: Explicit
Warnings: Underage
Chapters: 2
Words: 7,050
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26549083
Author URL: https://archiveofourown.org/
Summary: Osamu presents as an alpha and everyone assumes Atsumu will be one too. Until he presents as an omega.
Relationships: Miya Atsumu/Miya Osamu
Comments: 24
Kudos: 438
Collections: Anonymous





	1. Chapter 1

**Author's Note:**

> Please check the tags before reading!! This is a super self-indulgent fic that's been sitting in my WIPs for months... Head empty only thoughts of miyacest now.

When Osamu presents as an alpha, everyone assumes Atsumu is one too. When Osamu’s first rut hits, his mom takes Atsumu away for a few days, worried that his rut will trigger Atsumu’s; two teenage alphas in rut under one roof spell destruction. Both of their parents are betas, unaffected by his rut. His father leaves a black box outside the twins’ bedroom with some water and snacks. When Osamu opens the box, his face _burns_ ; it’s a fleshlight. He shoves it back into the box and tosses the box under his bottom bunk, confident he can sleep off the rut.

By evening, it becomes apparent that Osamu _cannot sleep off a rut_. He’s already jacked it more times than he can count, but he’s still unsatisfied. Even the fucking fleshlight doesn’t help. At some point, his sheets become gross, so he hops up onto Atsumu’s bunk; he’ll have time to wash the sheets later. Flopping onto the bed, he’s assaulted by Atsumu’s scent. He’s never noticed it before, but Atsumu’s scent is _nice_. Atsumu’s scent is predominantly cinnamon, but now Osamu can smell underlying notes of something sweet like _honey_ , and god he aches. He closes his eyes, fists his cock, and imagines Atsumu, mouth open, ready for Osamu’s cock. Fuck, Atsumu would be such a fucking tease too; he’d want to give little licks before finally taking Osamu’s cock. Just imagining fucking Atsumu’s mouth has Osamu coming into his fist. From then on, thinking of Atsumu brings Osamu the most relief. The reality that he masturbated to his _twin brother_ doesn’t hit him until he’s washing their sheets after his rut is over. Fuck. But he doesn’t have time to really think about _what the fuck happened_ because Atsumu and their mother arrive home, Atsumu loudly announcing their entrance.

“M’hungry! ‘Samu, make me somethin’!” Atsumu shouts from the kitchen. He hears his mother say something in response to Atsumu’s demand and counts to ten in his head before making his way to the kitchen. It’s fine. Everything’s fine. Just be normal. His mother’s nowhere to be found, most likely unpacking, and Atsumu’s digging through the refrigerator but closes the door when he hears Osamu approach.

“Make yer own food ‘Tsumu,” Osamu greets him. Atsumu pouts and Osamu sighs before pushing Atsumu out of the way so he can open the fridge. He grabs ingredients to make yakisoba. Atsumu’s eyes light up as he figures out what Osamu’s making.

“I want chicken!”

“I never said I was going to make you anything,” Osamu replies, but he does grab the chicken out of the fridge as well. He and Atsumu prepare the ingredients together, side-by-side. Osamu nearly slices his finger open when Atsumu leans close and sniffs him.

“Did ya get new cologne?” Atsumu leans into Osamu’s space even further and Osamu tries not to think about how long Atsumu's lashes look from this close, _were they always that long and pretty?_ With Atsumu close, Osamu gets a good whiff of Atsumu’s scent, and he feels his body flush with the memory of his rut.

“No? Why? Sayin’ I smell?” Osamu frowns, trying to shove Atsumu away. He can’t focus with Atsumu so close.

“I didn’t say that! Ya smell different, that’s all,” Atsumu pouts. “Kinda nice.”

* * *

News of Osamu presenting as an alpha spreads through school like wildfire. Honestly, Osamu doesn’t care, but now he’s getting confessed to almost daily. It’s nothing more than an annoyance since Atsumu complains about Osamu’s sudden rise in popularity.

“Ugh, I’m obviously the better twin! Why is everyone confessin’ to ‘Samu?!”

Their coach had been wary of Osamu’s return, expecting Osamu to fight against the third-years’ leadership, but Osamu submitted without a fight. Everyone laughed it off, teasing Atsumu for being the more difficult twin. The team has just finished practice and they’re waiting for Kita to finish his routine before they head out together. Suna and Aran are listening to Atsumu rant about Osamu’s return.

“Anyway, s’not like being an alpha makes ya any better at volleyball. ‘M still the best,” Atsumu concludes. Osamu rolls his eyes and continues eating his cookies. “Where’d ya get those? Share with me ‘Samu!”

“Some girls from home ec gave ‘em to me,” Osamu replies, popping another cookie into his mouth. Osamu’s answer sets Atsumu off again, much to Suna’s amusement. The cookies aren’t the best, but it’s food and he can appreciate the effort that went into making them.

“Hurry up and give me one ‘Samu!” Atsumu whines. Osamu rolls his eyes before grabbing a cookie to hand to Atsumu, but Atsumu has his eyes closed and mouth open expectantly. Immediately, Osamu thinks back to his rut, but his fantasy is nothing compared to the real Atsumu with his mouth open, waiting for Osamu. Before his thoughts can get out of hand, he quickly shoves a cookie into Atsumu’s mouth. Atusmu chokes in surprise and Osamu gets up before Atsumu can retaliate and mentally files away the sound of Atsumu choking for later.

* * *

“They weren’t as good as yours,” Atsumu mentions randomly as they walk home. It’s just the two of them now, the others have already gone their separate ways.

“What?”

“The cookies. They weren’t as good as yours,” Atsumu repeats. Osamu looks over at Atsumu to see him blushing. Cute. Hearing Atsumu praise his baking ignites a warm feeling. _H_ _is mate is pleased with his food. His mate?_ Osamu immediately shuts down that train of thought. _Nope._

“So you shouldn’t date the girl who gave you the cookies. Because they’re not good enough,” Atsumu rambles on.

“Tch. I wasn’t thinkin’ of datin’ her or anything, just took the cookies ‘cause she gave ‘em to me,” Osamu huffs. Too busy trying to block out thoughts of Astumu as his mate, Osamu misses the pleased look on Atsumu’s face.

* * *

Atsumu is invited to the National Youth Training Camp, but Osamu is not. Atsumu becomes nearly unbearable in his glee, spouting his “better twin” nonsense more than usual. The third years congratulate Atsumu on being invited, inflating his ego even more. In the end, it falls to Kita to rein in Atsumu.

“Atsumu, make sure you behave while you’re there. You’re representing Inarizaki,” Kita reminds him.

“Of course Kita-san! I’ll be on my best behavior!”

“As if. You wouldn’t know how to behave properly if your life depended on it,” Osamu smirks. Atsumu glares and grabs the front of Osamu’s shirt.

“Yer just jealous I got invited, and you didn’t!”

Osamu feels his hackles rise at Atsumu’s words. He knows he doesn’t love volleyball like Atsumu does, but something about what Atsumu says insinuates that he’s leaving Osamu behind and that’s unacceptable. Osamu growls and roughly grabs the lapels of Atsumu’s jacket.

“Watch yer mouth, ‘Tsumu,” Osamu bares his teeth and brings to growl. He needs to shake some sense into Atsumu before Kita or the coach breaks them up, but shockingly, Atsumu relaxes in Osamu’s grip and looks away, flushing. Surprised at Atsumu’s show of submission, Osamu loosens his grip on Atsumu.

“Shoulda practiced with me harder if ya wanted to go with me,” Atsumu says, still not meeting Osamu’s gaze.

 _Ah._ Osamu, well-versed in Atsumu’s mannerisms, finally figures out what Atsumu wants to say. _I’ll miss you. Practice harder so we can go together._ “Tch. I’ll practice better without you here, and I’ll be the one invited next year.” Osamu finally says. _I’ll miss you too._

“Ugh. Why can’t you guys say you’re gonna miss each other like normal people?” Suna groans. Osamu and Atsumu both turn to glare at Suna.

“Who’s gonna miss this asshole?” The twins say in unison, but before they can start arguing again, Kita steps in.

“Atsumu, you haven’t presented yet, have you?” Atsumu looks at Kita in confusion. “No? But ‘Samu’s an alpha so I’m one too.” It’s almost unheard of for twins to present differently.

“Kita-san, are you worried about Atsumu’s rut?” Aran asks. Osamu grimaces at the memory of his own rut. Kita hums in response, studying both Osamu and Atsumu before turning to Aran.

“It’s been weeks since Osamu’s rut, don’tcha think Atsumu would’ve had his by now?”

Atsumu begins to fidget, Kita’s words sinking in. If Atsumu isn’t an alpha, it’s almost guaranteed that he’s an omega and omega athletes aren’t very common and they’re generally subject to prejudice. Osamu can feel the anxiety radiating off of Atsumu and before he knows it, he’s growling at Kita.

Aran and Suna look at Osamu in surprise, Osamu hasn’t been aggressive at all since presenting. Osamu knows it’s wrong, that Kita’s just worried about Atsumu, but all he can think about is the bitter scent coming off of Atsumu. Kita puts his hands up in a placating manner, but Osamu’s growls don’t stop.

“Osamu, I’m not saying there’s anything wrong with Atsumu; I just think he should be careful.” Even with Kita’s calmand rational explanation, Osamu can’t stop the itch to fight Kita. _H_ _e upset Atsumu._

Atsumu, the source of all the trouble, ends up being the one to calm Osamu down. “S’fine ‘Samu! Quit yer growlin’, Kita-san already explained himself, he’s jus’ worried,” Atsumu grabs Osamu’s arm. Atsumu’s touch grounds Osamu slightly, but he can’t tear his eyes away from Kita until Atsumu starts really whining. “‘Samu! Yer embarrassing us!” Osamu tries to focus on Atsumu’s touch, and he feels himself calm down slightly; if Atsumu can complain about him, then he’s not thinking about what Kita said. “Hurry up and apologize to Kita-san!” It’s the wrong thing to say, and Osamu is back to baring his fangs at Kita.

“It’s alright Atsumu, I think Osamu’s worried for you too. Maybe you should take ‘im home now.” Kita suggests. Atsumu apologizes (which almost results in Osamu lunging for Kita) _how dare Atsumu submit to him_ \- before excusing Osamu and himself. He manages to drag Osamu away, mostly due to Osamu’s reluctance to be separated from Atsumu. Atsumu’s upset, but he’s upset at Osamu and that makes things even worse. Atsumu silently grabs their things from the locker room before grabbing Osamu’s arm and hauling him outside to walk home.

Atsumu being bratty and mouthing off when he’s upset, Osamu can handle. But Atsumu silently fuming puts Osamu on edge. They’re about to pass the convenience store and Osamu has an idea about how to pacify Atsumu (hopefully). He gently pulls his arm out of Atsumu’s grasp. “Hold up, I wanna get somethin’ from the store.” He doesn’t wait to hear Atsumu’s protests and enters the store to quickly grab a few snacks, including two of the premium pudding cups. When he exits the store, Atsumu’s waiting for him with his arms crossed, clearly irritated. Osamu’s surprised Atsumu didn’t just go ahead without him and doesn’t argue when Atsumu grabs his arm to continue dragging him home, even if there’s no reason for Osamu to run away.

Dinner’s an awkward affair with Atsumu clearly ignoring Osamu. His mother sends Osamu the “be a good older brother” look. He waits until it’s Atsumu’s turn to shower before putting his plan into motion; he tidies up their room and sets the pudding cup on Atsumu’s desk. He sits on his bunk, quickly grabbing a book to pretend to read when he hears Atsumu enter.

Atsumu’s wearing one of Osamu’s old shirts and his carefully prepared not-apology speech goes out the window. Before, Atsumu wearing his clothes would piss him off, but now… Seeing Atsumu in his clothes makes him think about how _cute_ Atsumu looks and soothes his growing possessiveness. Atsumu pays no mind to Osamu, eyes landing on the pudding cup. He gleefully snatches the cup and peels back the lid, devouring the treat. Osamu exercises every ounce of self-control he has as Atsumu obscenely sucks the caramel off the spoon.

After Atsumu finally finishes, he turns to Osamu. “M’not cheap enough to forgive you with jus’ this, ya know,”. Osamu rolls his eyes; Atsumu _would_ be the type to be greedy when Osamu’s trying to apologize. “‘Sides, ya should be apologizin’ to Kita-san.”

Osamu sighs, “I’ll apologize to him on Monday, I swear.” Atsumu looks Osamu intently before nodding. Atsumu starts rummaging around their room, packing for the training camp. Osamu abandons the pretense of reading and watches as Atsumu haphazardly throws his things into a duffel bag. “Oi, that’s my sweater you just threw in there!”

Atsumu turns to Osamu and pouts, “Don’t be so stingy ‘Samu! I’m just borrowing it!” Osamu closes his eyes and summons patience from god knows where before giving in.

“Fine. But don’t lose it or anything, it’s one of my favorites.” He watches as Atsumu removes the sweater from the pile, gently placing it to the side.

“I’ll take care of it, geez!” Atsumu sounds annoyed, but the care in moving the jacket says otherwise. Osamu grabs his sweater back while Atsumu’s busy, making sure the sweater carries Osamu’s scent before returning it to its place. He tells himself it’s so Atsumu will remember who the sweater actually belongs to, but he knows it’s a lie.

Eventually, Atsumu finishes packing, but instead of climbing onto his own bunk to sleep, he flings himself onto Osamu’s. “The hell ‘Tsumu? Go sleep on your bed!”

Atsumu doesn’t bother responding to Osamu, somehow worming his way under the blanket instead. Osamu can think of at least a dozen reasons why Atsumu should go sleep on his own bunk, especially since it’s a tight fit on the bed, and Osamu really really shouldn’t be so close to Atsumu with all the shit he’s trying to sort out.

“C’mon ‘Samu! Just let me sleep here tonight, I’m tired…” Atsumu whines. Osamu seriously considers shoving Atsumu off his bed, but Atsumu’s eyes are already closed and Osamu’s stupid alpha instincts are having a field day because Atusmu is wearing _his_ shirt and in _his_ bed. Osamu sighs and resigns himself to getting up to sleep in Atsumu’s bunk. Before he can crawl over Atsumu though, Atsumu stops him, “Jus’ stay and sleep here with me, s’cold and you’re warm.”

Once again, Osamu finds himself helpless to Atsumu’s whims. “Fine, but I’m sleeping on the outside, I always wake up before you,” Osamu maneuvers them until he’s satisfied, with them back to back. It _is_ warmer in bed with Atsumu, and Osamu can feel himself drifting off until Atsumu moves, more accurately starts trying to steal even more of the blanket. Osamu turns to tell Atsumu off but ends up coming face to face with Atsumu who is too close. “Stop hogging all the blanket ‘Tsumu!” Osamu hisses, trying to avoid eye-contact with Atsumu.

“I’m still cold! Just let me-” Osamu goes still as Atsumu snuggles up to him, throwing a leg over his hip and burrowing his face into the crook of Osamu’s neck. Atsumu sighs contentedly before his breathing begins to even out. Osamu can feel his heart beating rapidly and he wants to die. He feels exhausted, physically, and mentally. He’s tired from fighting with Kita and Atsumu. He’s tired of fighting his instincts. For now, Osamu lets himself relax and enjoy Atsumu’s presence. And if he turns and presses a gentle kiss to the top of Atsumu’s head, then no one has to know.

* * *

A week without Atsumu should be a blessing. But it’s not. At times the house feels too quiet and he feels restless. He can’t seem to settle unless he’s at home where he’s stolen Atsumu’s blanket and pillow to add to his bed, their combined scents soothing him.

He apologizes to Kita and suffers endless teasing from Suna about it. “I wish I had gotten it on video. You, growling at Kita-senpai of all people!” Suna laughs. Osamu rolls his eyes and continues his cooldown stretches. “Ah, looks like you’ve got some admirers here today,” Suna tilts his head towards the entrance of the gym. There’s a group of girls and the bags of sweets he spies leave no room for doubt as to who they’re here for.

“At least Atsumu’s not here to complain,” Aran sighs. “I’m sure the number of admirers will go up once Atsumu presents too.” Osamu feels a surge of possessiveness at Aran’s words. _Atsumu is his_. Atsumu’s absence hasn’t made any of Osamu’s feelings disappear, if anything they’re growing day by day much to his chagrin.

“Have you heard from him at all, Osamu?” Ginjima asks as they all leave the gym together. Wordlessly, he shows them the endless stream of photos Atsumu has been sending him. There’s quite a few photos of the food Atsumu’s been eating and some pictures of him with other players he’s met. “Holy crap, that’s Ushiwaka with him!” The others crowd around Ginjima to see the photo. It’s a selfie of Atsumu and Ushijima; Atsumu grinning and holding up a peace sign next to Ushijima who is surprisingly holding up a peace sign as well.

“It’s no surprise he got invited as well considering his talent,” Aran muses. “Even in this photo, he looks intense.” Seeing the picture always bothers Osamu just a little. Why does Atsumu look so close to Ushijima? Atsumu’s probably annoying him anyway. He’s not jealous of Ushijima at all. If anything he’s probably jealous of Atsumu for being able to play with such a talented player. _Yeah right._

“Look! There’s Sakusa too from Itachiyama! Oh man, he does not look happy to be in this photo with Atsumu…” That picture doesn’t bother Osamu as much; Atsumu’s got his arm slung around Sakusa, but Sakusa has a clear look of disgust and appears to be seconds from shoving Atsumu away.

“I’m glad Atsumu looks like he’s having fun though,” Kita says. “He comes back tomorrow, right?” Osamu nods.

“All the peace I enjoyed comes to an end,” He sighs. He’s absolutely not going to pick up Atsumu from the station. Atsumu would probably make him carry his stuff back. But if he happens to make curry udon tomorrow, one of Atsumu’s favorite wintertime foods, well that’s just a coincidence.

* * *

All thoughts of relaxing vanish from his mind when he gets home. His parents seem to be in a frenzy.

“Ma? What’s going on?” Osamu asks cautiously. His mother looks distressed and searching for something.

“I made a list of things we’ll need to get on our way, but I can’t remember-”

“Here! I’ve got it!” His father shouts.

“Oh Osamu, your brother’s presented at the training camp…” His mother sighs.

“Should I go stay with auntie for a while? You don’t want his rut triggering mine right?” Osamu sighs. It’d be a pain, but it’s only fair.

“No honey, Atsumu’s gone into _heat_. He’s presented as an omega.”


	2. Chapter 2

**Summary for the Chapter:**

> Atsumu goes into heat.

**Notes for the Chapter:**

> Ahh thank you everyone for your kudos and comments!! The support means a lot to me!!! (இдஇ; )  
> Please enjoy this chapter!

On the day he’s supposed to leave for the training camp, Atsumu wakes up before Osamu. Atsumu wakes up slowly and reluctantly; he’s warm and comfortable, but the buzzing of Osamu’s alarm is annoying him. He remembers cuddling up to Osamu before falling asleep last night, in search of warmth, but sometime during the night, he’d switched sides and now Osamu’s spooning him from behind. Osamu’s got an arm wrapped around Atsumu’s waist and Atsumu can feel the warmth of his breaths against the back of his neck. Atsumu’s first instinct of course, is to shove Osamu awake so he can shut off the alarm and to rub in the face that he woke up after Atsumu… But he doesn’t. He knows something’s been bothering Osamu since his rut, and whatever it is is stressing Osamu out. Osamu frowns more often lately, even when eating his favorite foods and it’s not that Atsumu _cares_ , it’s just… If something is bothering Osamu to this extent, then it could affect his volleyball playing. And that’s all that matters. Not Osamu’s crisis, not Osamu’s _annoying_ admirers, not Osamu’s weird clash with Kita… Just volleyball.

Atsumu decides that he doesn’t need his own personal crisis right before the training camp, so the best thing to do is just not think about it. He settles back against Osamu and lets the sound of Osamu’s breathing lull him back to sleep. 

* * *

At the training camp, Atsumu becomes fast friends with Komori Motoya much to the disgust of Sakusa. He sets for _the_ Ushijima and watching him spike the balls Atsumu tosses for him sends a thrill up his spine. 

On the last night of the camp, he and Komori chat about upcoming Nationals during dinner. Atsumu pauses to snap a picture of his food before eating. “Ah, Atsumu-kun, why do you take so many pictures of your food?” Komori asks curiously. 

“I send ‘em to ‘Samu. I know it’ll piss him off when he sees all the good grub I get to eat,” Atsumu grins. It’s a half-truth; he really does send the pictures to ‘Samu to piss him off, but he also likes to think it’s his small way of _maybe_ motivating Osamu to practice harder, so they can go together next year. As much as he loves tossing to the hitters at the training camp, it’s not quite the same as with Osamu. Atsumu will never admit it, but he knows that Osamu is better than him skillwise. _Just a teensy bit_. But he doesn’t hunger for volleyball the way Atsumu does. And if seeing the food makes Osamu love volleyball a little more, then Atsumu doesn’t mind playing dirty and tempting Osamu with food, not if it means they get to play together more. 

“Disgusting, what, are you a child?” Sakusa glares from next to Komori. Komori sighs at Sakusa’s comments, but Atsumu’s used to them by now.

“I’m sure Atsumu-kun just misses his twin, right?” Before Atsumu can reply that _he absolutely does not miss Osamu_ , a tray is placed next to his. 

“Is this seat taken?” Ushijima asks. 

“Nope! Go ahead, Ushiwaka!” Atsumu pats the seat next to him. Ushijima sits down next to Atsumu and removes a pill container and pops a pill into his mouth before washing it down with water. 

“I wasn’t aware you were feeling ill, Wakatoshi-kun,” Sakusa comments warily.

“I’m not. These are to suppress my rut. I was due for one soon, but I’m taking these to delay them until after we win Nationals,” Ushijima explains as he prepares to eat. Before Atsumu and Sakusa can argue, Komori cuts them off. 

“Ah, I’m grateful I’m a beta. Sakusa, you had yours recently, so you’re not worried, right?” Sakusa nods as he continues to glare at Ushijima. Atsumu feels anxiety begin to build. _What if he goes into rut during Nationals?_ The suppressants can only be taken safely once an alpha has their first rut… _Or what if he is an omega, and he goes into heat?_

“Are you alright, Miya?” Ushijima places a hand on Atsumu’s shoulder. Surprisingly, the weight of Ushijima’s hand settles Atsumu. It makes him think of when Osamu slings his arm around him or puts his hand on the back of his neck when Atsumu gets too worked up. _Nothing to worry about._ Atsumu doesn’t trust himself to speak yet but flashes a smile at Ushijima. Ushijima studies Atsumu a moment longer before returning to his food. Atsumu feels himself growing warm and turns back to Komori. 

Komori has his eyebrows raised but thankfully doesn’t call Atsumu out on the strange interaction. “So do you miss your twin?” He asks instead. 

_Yes_. But Atsumu doesn’t say that. He laughs before telling Komori, “Nah. He’s a scrub for not makin’ it, who’d miss a scrub?” Komori doesn’t need to know that every night he wears Osamu’s soft fleece sweater to sleep. He doesn’t need to know that Atsumu misses Osamu’s homemade food, that Atsumu feels the silence in the room is suffocating without Osamu’s soft snores at night. _That’s just for Atsumu to know._

“Miya, come to Shiratorizawa.” Ushijima interrupts. Komori’s mouth opens in surprise and Atsumu can see Sakusa’s eyes widen in surprise too. For once, Atsumu feels at a loss for words. “You’re a skilled setter,” Ushijima continues. “Be my setter and devote yourself to me.” 

Atsumu doesn’t know how he hasn’t combusted into a ball of flames. How does Ushijima say _those things_ with such a straight face? Fortunately, Sakusa saves Atsumu from the embarrassment of having to respond. 

“It’s no use Wakatoshi-kun. Miya’s second-rate compared to your Oikawa. He’d be a terrible replacement,” Well, Sakusa’s save ends up being more like an insult, but it snaps Atsumu out of his shock. 

“Who’re ya callin’ second-rate!? S’not like I see this Oikawa here either!” Atsumu snaps. He turns to Ushijima, “Anyway _Ushijima-san_ , I ain’t a replacement for nobody. We’re gonna win Nationals and you’ll be beggin’ to come to Inarizaki!” Atsumu grabs his tray and storms off. 

It’s not like he was considering Ushijima’s offer seriously, but nothing gets under Atsumu’s skin more than being thought of as _a second-choice_ , some sort of fucking consolation prize. He showers before collapsing into his bed. He’s tempted to call Osamu and vent, but knowing Osamu, he’d probably insult Atsumu more. Instead, he burrows under the covers and lets the scent of Osamu’s sweater soothe him before he drifts to sleep. 

In the middle of the night, Atsumu wakes up sweating. He feels feverish and his body aches all over. It’s a different kind of ache from his muscles aching from a grueling practice. He feels anxiety creep in, a sense of helplessness washing over him. _He needs Osamu_. He’s tempted to call his twin, even if it’s just to hear his voice, but his heart sinks when he realizes the time. It’s a little past midnight, there’s no way Osamu would pick up. He takes some deep breaths before crawling back into bed. Despite feeling feverish, he crawls under the covers and brings his sleeves up to his nose to inhale; Osamu’s scent comforting him. _Everything’s fine. This is just some weird nighttime sweats thing. Tomorrow I’m going home to see Osamu again._

* * *

Atsumu wakes up to pounding on his door. He still feels like shit and the noise makes his head hurt even worse. He can barely open his eyes to glare at the door. The knocking doesn’t subside, if anything it grows more frantic, so Atsumu manages to drag himself out of bed, annoyed. His body feels heavy, but he somehow manages to open the door to a worried Komori, with Ushijima and Sakusa flanking him.

“Atsumu-kun! You missed breakfast, are you alr-” Before Komori can finish, Atsumu feels himself swaying and his legs give out. Ushijima moves forward and catches him. Atsumu moans at how cool Ushijima's skin feels against his own. “Woah, is he alright?” Komori frets. 

“Is he sick? Keep him away from me,” Sakusa hisses. Ushijima tries to adjust Atsumu in his grip, but Atsumu whimpers at the movement. He tries to get closer to Ushijima, seeking relief. Ushijima’s eyes widen when he becomes aware of the sweet scent emanating from Atsumu.

“Komori-kun. I think you should stay here with Miya-kun while Sakusa and I go get help,” Ushijima states calmly. He tries to pass Atsumu to Komori, Atsumu protesting the loss of contact. “It’s a bad idea for Sakusa and I to be near him right now.”

“What’s wrong with him?” Komori asks warily, trying to support Atsumu’s weight. Atsumu’s still weakly trying to reach for Ushijima and making distressed sounds.

“He’s in heat,” Ushijima replies before jogging away. Sakusa and Komori look at each other in shock before Sakusa goes after Ushijima. 

Atsumu doesn’t remember much after that. He remembers Komori staying with him and trying to reassure him, but everything else is a blur. His parents are contacted and he’s whisked away to the training camp clinic to wait for them. He whines when the nurse asks him to remove Osamu’s sweater, immediately clutching it to his chest while she tries to cool him down with ice packs wrapped in towels. 

His parents pick him up and he’s crammed into the back seat of their car with what appears to be assorted bedding and pillows, _nesting supplies_ , his mom tries to explain to him. He sleeps fitfully on the way home, his parents speaking in urgent hushed tones amongst each other. 

Osamu isn’t there to greet him when he gets home, and he’s not in their room either. Atsumu can feel his distress increasing, _where’s Osamu?_ But, his mom ushers him into their room with the mountain nesting supplies before he can ask. She hands him a box, tells him it’s to _help with his heat_ ; it’s a fucking _dildo_ of all things. Atsumu doesn’t have the energy to even be embarrassed. He’s not sure how to start building his “nest”, but before he knows it, instinct has taken over and he’s pulled the blankets off Osamu’s bunk and used the new pillows and bedding to make a soft pile in the middle of their room. Satisfied and sleepy once again, Atsumu curls up in the center, pulling Osamu’s blanket over himself.

* * *

Osamu’s not a coward. Really. He just made a strategic decision to go to the convenience store when Atsumu was due to come home. When he does arrive home, Atsumu is thankfully nowhere in sight, probably holed up in their room. Resigned to sleeping on the couch until Atsumu’s heat is over, Osamu heads to the kitchen to put away his snacks but walks in on his parents arguing.

“We can’t take them with us, with Atsumu in heat!”

“But it could be serious enough that we need to go up there, just in case!”

“What’s going on?” Osamu asks.

“It’s your grandmother… She fell and broke her hip. She’s due for surgery tomorrow and we want to go up there to be with her, but with Atsumu in heat…” His mother trails off. 

“I can stay and take care of ‘Tsumu,” Osamu finds himself saying. His parents look at him surprised. “I mean, he just needs water and food, right?” _And a good fucking_. His parents share a look before silently coming to a decision. 

“Thank you Osamu, we’ll leave tomorrow morning. We’ll give you enough money for a week, just in case. We’ll contact the school and let them know what’s going on. With Atsumu as an omega, we wouldn’t feel comfortable with someone outside the family looking after him, but if you’re here, it should be alright,” His mom smiles. His parents have absolutely no reason to suspect that Osamu harbors _feelings_ for Atsumu, and it’s not like he’d ever act on them anyway, so everything’s fine. _Really._

His parents leave first thing in the morning, leaving Osamu alone with Atsumu. He hasn’t heard a peep from Atsumu since coming home, but passing by their bedroom door makes Osamu’s mouth water, with Atsumu’s scent growing stronger. He makes kabocha miso soup for them both, bringing a tray for Atsumu to their room. He tells himself he’ll just knock and leave the food, but he finds himself opening the door. Osamu is immediately met with the heady scent of honey and cinnamon. In the center of the room is Atsumu in his nest, sleeping peacefully. He’s wearing Osamu’s sweater with the blankets kicked away, showing very clearly that he’s wearing Osamu’s sweater and _nothing else._ Osamu’s mouth goes dry at the thought, but he busies himself putting the tray of food on the desk. It’s a testament to how much Atsumu trusts Osamu that he hasn’t woken up, most omegas would be on alert if an alpha was in such close proximity to their nest. _But Atsumu is Osamu’s omega so he’s relaxed_. 

Osamu resists the urge to climb into the nest with Atsumu, instead choosing to clear his throat loudly. Atsumu frowns at the noise and buries his face into _Osamu’s pillow_. Osamu’s tempted to take a picture of how cute Atsumu looks but decides against it, this sight is for his eyes only. He clears his throat louder, and this time Atsumu cracks an eye open. “‘Samu?” 

“Wake up ‘Tsumu, and quit yer droolin’. That’s my pillow,” Osamu greets him. Atsumu bolts up at his words and checks the pillow for drool. Finding none, he glares at Osamu. “I’m just here to drop off food and water. Jus’ leave the tray outside the door when yer done,” Osamu instructs. Now that Atsumu’s awake, Osamu can leave, but his body won’t cooperate. Atsumu studies him intently, before smirking. 

“Don’t wanna,” _Of course Atsumu would be a brat_. Osamu feels a headache coming on. He’s about to tell Atsumu he can starve, but Atsumu interrupts, “I’m too tired. Feed me ‘Samu,”. He finishes by scooting over and patting the space next to him in the nest. Osamu stares, there’s no way Atsumu doesn’t know the implications of an omega inviting an alpha into their nest, twins be damned. All Osamu has to do is tell Atsumu to fuck off and leave the room. They can pretend this never happened, but his body feels heavy, and imagining Atsumu inviting any other alpha into his nest infuriates Osamu. As if sensing Osamu’s hesitation, Atsumu stretches slightly causing Osamu’s sweater to ride up _just a bit_ , but it’s enough to seal the deal. Osamu knows that once he climbs into the nest, there’s no going back. 

Osamu grabs the tray and sets it close to the nest before he climbs in. Once he settles in, he finds himself face to face with a smug Atsumu. Feeding Atsumu should be a simple task, but Atsumu teases Osamu; moving closer to Osamu under the pretense of not spilling and forcing Osamu to hand feed him the orange slices as well. Osamu’s gaze darkens as Atsumu grabs Osamu’s wrist to suck the juice off Osamu’s fingers. _Was Atsumu always like this? No. This was only for Osamu. No one else_. Once finished, Osamu sets the tray far from the nest and turns back to Atsumu; he’s flushed and looking at Osamu coyly, there’s no denying they’re both hard. 

“Gonna take care of me ‘Samu?” _Fuck_ . Osamu hauls Atsumu into his lap and into a kiss. It’s a little clumsy and awkward, Osamu’s sure it’s both of their first kisses, but the thought of being Atsumu’s _first_ spurs Osamu on. Atsumu bites down on Osamu’s bottom lip and Osamu growls warningly. Atsumu whimpers and swipes his tongue over it apologetically. Osamu’s hands stroke Atsumu’s sides underneath the sweater before settling on Atsumu’s hips to squeeze roughly, Atsumu moaning loudly. They break apart, the sound of their harsh breathing filling the room. Osamu trails a series of kisses along Atsumu’s jaw, down to his neck. Atsumu tilts his neck to allow Osamu easier access, and Osamu hums, pleased. It would be so _easy_ to just bite down and mark Atsumu as _his_ , for the rest of the world to know. Osamu manages to resist, instead pressing a soft kiss to Atsumu’s scent gland. Atsumu pushes weakly at Osamu and he growls in response. 

“‘S’hot. Wanna take the sweater off. You too ‘Samu… Take yer shirt off…” Atsumu whines. Osamu places one last kiss on Atsumu’s neck before taking off his shirt. He hesitates for a moment before kicking his sweatpants and briefs off as well. Satisfied, he turns back to Atsumu, catching him staring. 

“Well? Good enough for ya?” Osamu smirks. Atsumu flushes before climbing back onto Osamu’s lap, pulling Osamu in for another kiss. Without their clothes in the way, every point of contact feels electric. Atsumu shamelessly grinds against Osamu, whining when Osamu breaks the kiss. “Fuck ‘Tsumu, yer so fucking needy…” Atsumu moans in response before grabbing Osamu’s hand and bringing it to his cock. 

“‘Samu… _Please_ …” Hearing Atsumu ask _nicely_ sends a thrill through Osamu; he wastes no time in getting his hand around both their cock; Atsumu ruts into his grip instinctively. Osamu moans at the friction and starts to pump. They come embarrassingly fast, Atsumu chanting Osamu’s name while Osamu comes with a grunt. Osamu thinks of wiping the come on one of the blankets, but Atsumu snatches his wrist and licks it off. Osamu can feel himself harden again, imagining the wet heat of Atsumu’s mouth around his cock instead. Atsumu finishes and Osamu pulls him into another sloppy open-mouthed kiss. 

When they break apart next, they’re both hard again. Atsumu pulls away and reclines against some pillows, his legs spread invitingly. “‘Samu, don’t tell me that’s all ya got?” Atsumu strokes his cock languidly, maintaining eye contact with Osamu. “Arent’cha gonna come and fill me up?” 

“Where’s the lube ‘Tsumu?” Osamu asks, breaking eye contact to look around the room. 

“Don’t need it, m’wet enough,” Atsumu whines. Osamu wills himself to remain somewhat level-headed and spots the lube near the edge of their bed. Thankfully, there’s a box of condoms nearby and he grabs a few. Atsumu frowns when Osamu rips the foil packet, rolling the condom on. “Wanna feel you,” Atsumu complains. “And I told you we don’t need the lube.”

“Don’t wanna hurt you for your first time,” Osamu explains.

“Who says it’s my first time?” Atsumu responds cheekily. Osamu pins Atsumu down in an instant, growling. 

“No one else ‘Tsumu, yer mine. Tell me, has anyone else seen you like this?” Osamu holds Atsumu’s gaze and a hand makes its way to Atsumu’s throat, stroking his scent gland. Atsumu bares his throat in submission, but there isn’t a trace of fear in him. Atsumu just continues to smirk, looking like a cat that ate the canary.

Atsumu waits for a beat before responding, “No. Just you ‘Samu. No one else.” Osamu feels himself relax slightly and leans down to press a kiss to Atsumu’s neck. 

“Tell me who you belong to ‘Tsumu,” Osamu demands, one hand trailing down to play with Atsumu’s nipples. 

“Ngh…” Atsumu resists, arching into Osamu’s touch instead. Osamu trails his hand down, past Atsumu’s cock, and down to his hole. There’s slick and Osamu’s sure there’s not much resistance, but he doesn’t push in, _not yet_. He strokes Atsmu’s hole instead, applying the barest hint of pressure. 

“C’mon ‘Tsumu. Tell me who you belong to,” Osamu repeats. All of Atsumu’s cockiness is gone; he whimpers and tries to get Osamu to give him more. “Be a good little omega and tell me who your alpha is, “ Osamu tries again. 

“M’yours ‘Samu. Yer my alpha,” Atsumu finally moans. Osamu rewards him by inserting a finger, and Atsumu sobs in relief. Osamu planned to go slow, but with how riled up he is, it’s impossible. He adds a second finger, then a third, and before long he’s lined his cock up to Atsumu’s hole. When Osamu _finally_ pushes in, he groans at how _hot and tight_ Atsumu is, even with the prep. He grits his teeth as he waits for Atsumu to adjust before moving. Osamu starts with slow shallow thrusts, but with Atsumu’s encouragement, he sets a rough pace.

“Fuck, you were made for me, weren’tcha?” Osamu groans as Atsumu continues to sob for Osamu to give him _more_. 

“‘Samu, _please_ , I’m about to-!” Atsumu warns. Osamu doesn’t falter; he feels himself coming close as well, and he can’t think of anything better than them finishing together. 

“S’alright ‘Tsumu, just let go, I’ve got you.”

Atsumu always thought people who said sappy things about finding their other half, their _soulmate_ , were stupid. But afterward, laying in the nest he made next to Osamu, Atsumu thinks maybe they were onto something. 

* * *

The week passes by in a blur; Atsumu and Osamu eat, sleep, fuck, and shower most of the time. Everything and nothing has changed. Atsumu’s still spoilt, stealing Osamu’s food when they eat or demanding Osamu feed him, but he’s also softer in a way. He’s more physically affectionate with Osamu, lacing their hands together and resting his head on Osamu’s shoulder. Osamu doesn’t mind, but his favorite change is how shy Atsumu can become, especially when they fuck. It’s a side to Atsumu that’s just for Osamu. 

Of course, all good things come to an end. The morning before their parents are scheduled to return, Osamu gets an actual call from their mother. He takes the call in the kitchen while he makes breakfast. 

“Your grandma’s recovering really well Osamu! Your dad and I are so proud of you for taking care of Atsumu. We know it can’t have been easy and it might have been awkward, but you’re such a good brother,” His mother praises. _A good brother_. Osamu feels sick. 

He doesn’t remember much of what his mother says after, something about an appointment for Atsumu. He snaps out of his daze when Atsumu wanders into the kitchen. Atsumu’s heat is essentially over and they’re laundering the bedding and soiled clothes, but Atsumu appears to have stolen another sweater from Osamu’s closet. He sidles up to Osamu, head turned up expectantly, for a kiss. Atsumu looks happy and Osamu is going to have to ruin it. 

“‘Tsumu,” Osamu starts. Atsumu frowns, not receiving his kiss. “This was a mistake.” Atsumu’s eyes widen in confusion, and Osamu continues. “We… We just got caught up in yer heat. We can’t tell anyone about this, we’re gonna pretend it never happened.” He feels Atsumu tense up and he studies Osamu's face intently before Atsumu's confusion morphs to anger. 

“Liar, ya said I was yours!” Atsumu argues. 

“Did you really believe me ‘Tsumu!? We’re twins for fuck’s sake! What would mom and dad think!?” Osamu shouts. Atsumu flinches at Osamu’s tone, but refuses to back down. 

“I don’t care what others think… You said I’m yours and you’re mine!” 

“I just got caught up in your heat; I didn’t mean any of that shit,” Osamu lies. Hurt fills Atsumu’s face and Osamu wants to take everything he said back, none of it is true, but he keeps his mouth shut. Atsumu’s eyes begin to fill with tears and Osamu feels his chest constrict. Atsumu whirls and storms out of the kitchen and Osamu hears the door to their room slam.

He leaves food outside the door for Atsumu. The food goes untouched for the rest of the day.

Osamu sleeps on the couch that night.

**Notes for the Chapter:**

> Still have lots more planned...Thinking of doing "interludes" with POV from other characters! Like maybe Ushijima and Sakusa after Atsumu presented. We'll see! Writing is still hard _(┐「ε:)_

**Author's Note:**

> I have a lot more planned (peep those tags), but writing.. is hard... ∠( ᐛ 」∠)＿  
> Yes I am (gently) pushing my ushiatsu/sakuatsu agenda, but this is mainly osaatsu ok... Just let me indulge in my everyone/atsumu dreams.


End file.
